Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite faces The MEA is sandwiched between a pair of non-porous, electrically conductive elements or plates which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings formed therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts.
The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are typically bundled together to form a fuel cell stack and are commonly arranged in electrical series. Each cell within the stack includes the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2) or air (a mixture of predominantly O2and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluorinated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. As such these MEAs are relatively expensive to manufacture and require certain conditions, including proper water management and humidification and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
The electrically conductive plates sandwiching the MEAs may contain an array of grooves in the faces thereof that define a reactant flow field for distributing the fuel cell's gaseous reactants (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective cathode and anode. These reactant flow fields generally include a plurality of lands that define a plurality of flow channels therebetween through which the gaseous reactants flow from a supply header at one end of the flow channels to an exhaust header at the opposite end of the flow channels.
Covering the reactant flow fields is a diffusion media serving several functions. One of these functions is the diffusion of reactant gases therethrough for reacting within the respective catalyst layer. Another is to diffuse reaction products, namely water, across the fuel cell. Additionally, the diffusion media must conduct electrons and heat between the catalyst layer and bipolar plate. In order to properly perform these functions, the diffusion media must be sufficiently porous while maintaining sufficient strength. Strength is required to prevent the diffusion media from overt compression within the fuel cell and also from impingement of the diffusion media into the channels of the flow field. Overt compression of the diffusion media results in reduced diffusion capacity. Impingement of the diffusion media into the channels results in reduced flow capacity through the channels and high pressures therein as well as possible loss of electrical contact between the diffusion media and the catalyst layer adjacent to the channels.
Traditional diffusion media have sought to provide the required strength characteristics through the introduction of stiffer materials, such as metal mesh, within the plane of the diffusion media. Such solutions limit flexibility in shipping and manufacture of the fuel cells and introduce corrosion and contact resistance issues.